In the related art, various techniques for constraining a rider by mounting an airbag apparatus on a vehicle such as a motorcycle or the like are known. For example, in a motorcycle, a technique for deploying and inflating an airbag stored in a housing mounted to a vehicle body frame by inflation gas, thereby constraining a rider in case of front collision is known (for example, see Japanese Unexamined Patent Application Publication No. 2002-137777). In this technique, a possibility to secure a wide constraining area by the airbag is proposed. However, in an airbag apparatus to be mounted to a vehicle having such a structure that the periphery of the rider is opened like the motorcycle, constitution of an improved technique effective when constraining the rider radically by the airbag in case of front collision is highly requested.